As a hydromechanical frequency analyzer of complex environmental sounds, the mammalian cochlea is capable of processing amplitude, phase, and frequency-varying sounds in a wide dynamic range and with a high degree of frequency and time resolution. These capabilities are due to the remarkable sensitivity, nonlinearity, and sharp tuning of the cochlea. To understand how the cochlea works, a cochlear amplifier (CA) has been proposed by some investigators, which amplifies the basilar membrane (BM) vibration evoked ! by low level sound and enhances mechanical frequency selectivity. The CA is proposed to work by obtaining energy from the outer hair cells (OHCs) on a cycle-by-cycle basis, resulting in enhancement or suppression of the BM vibration through either positive or negative feedback, depending on frequency. The aim of this study is to experimentally test if the CA exists through the use of our newly developed signal processing method and to study the feedback hypothesis of the CA by observing the phase relationships of the BM vibrations evoked by acoustical and electrical stimuli. The following specific aims will be investigated: i) whether the long delay component (LDC) of the EEOAE comes from the characteristic frequency place on the BM, ii) whether the amount of LDC acoustical energy changes with cochlear sensitivity, and iii) whether the CA gain is numerically greater than one and whether it is dependent on the level of electrical current or the EEOAE sound pressure. Finally, iv) the amplitude and phase of the BM velocity evoked by acoustical and electrical stimuli will be measured as functions of frequency and intensity. The phase relationship beneath the OHCs will be used to test the frequency and level dependent feedback mechanism of the CA. Because of the noninvasive approach, the wide frequency response feature of the EEOAE, and the inherent self-testing property of the method, the gain measurement of the CA in this proposed study is expected to be accurate and reliable. The project provides fundamental experimental data both for advancing our understanding of cochlear mechanisms and for using multiple component analysis method as an important tool for auditory research and clinical diagnosis.